The present invention relates generally to feature measurement in scanning electron microscopy, and more specifically to apparatus and methods for optimizing image quality by controlling the buildup of charge on a sample. The present invention may also be applied to feature measurement and image enhancement in similar instruments.
FIG. 1 is a diagrammatic representation of a conventional scanning electron microscopy configuration 100. As shown, a beam of electrons 102 is scanned over a sample 104 (e.g., a semiconductor wafer). Multiple raster scans 112 are typically performed over a small area 114 of the sample 104. The beam of electrons 102 either interact with the sample and cause an emission of secondary electrons 106 or bounce off the sample as backscattered electrons 106. The secondary electrons and/or backscattered electrons 106 are then detected by a detector 108 that is coupled with a computer system 110. The computer system 110 generates an image that is stored and/or displayed on the computer system 110.
Typically a certain amount of charge is required to provide a satisfactory image. This quantity of charge helps bring out the contrast features of the sample. Although conventional microscopy systems and techniques typically produce images having an adequate level of quality under some conditions, they produce poor quality images of the sample for some applications. For example, on a sample made of a substantially insulative material (e.g., silicon dioxide), performing one or more scans over a small area causes the sample to accumulate excess positive or negative charge in the small area relative to the rest of the sample. The excess charge generates a potential barrier for some of the secondary electrons, and this potential barrier inhibits some of the secondary electrons from reaching the detector 108. Since this excess positive charge is likely to cause a significantly smaller amount of secondary electrons to reach the detector, an image of the small area is likely to appear dark, thus obscuring image features within that small area. Alternatively, excess negative charge build up on the sample can increase the collection of secondary electrons causing the image to saturate. In some cases, a small amount of charging is desirable since it can enhance certain image features (voltage contrast) as long as it does not cause image saturation.
The excess charge remaining from a previous viewing or processing may therefore cause distortion. One solution used in SEM devices is to flood the sample with charged particles from a separate flood gun at a time separate from the inspection. This flooding equalizes the charge appearing across the sample, thus enhancing the voltage contrast images. One drawback to this flooding procedure is the need to move the stage including the entire sample to the area of the flood gun. In order to accomplish the flooding, the inspection must stop to permit movement of the sample to the area of the flood gun. This dramatically increases the overall time required for the inspection since movement and flooding of the sample may take ten minutes or more to complete. This produces an equally dramatic decrease in the throughput for the inspection process. Typically a full inspection of a sample will require hundreds of scan lines across the sample and the dissipation of charge may be required after only a few scan lines have been completed. The total time required for a sample to be inspected therefore is the sum of the separate intervals for charge dissipation (or precharging) and inspection. Consequently what is needed is a method or apparatus which will facilitate charge control on a sample without requiring the removal of the sample from the inspection beam or otherwise require the inspection beam operation to be interrupted.
As noted, voltage contrast imaging requires good control over the charge provided to the sample. The precharging step is important to provide a high quality voltage contrast image. Control over the amount of electrons emitted from the sample can affect the resolution of the image. Accordingly methods and apparatus which will provide fine control over the amount of secondary electrons reaching the detector will have a direct effect on the image quality.
Accordingly, the present invention addresses the above problems by providing apparatus and methods for controlling charge distribution on the sample (e.g., so as to improve image quality). Charge is controlled by providing an flood beam coupled to move in tandem with an inspection beam. The flood beam dissipates charge in a first area, while the inspection beam inspects a second area of the sample. In one implementation, the relative motion between the sample and the coupled inspection beam and flood beam is controlled so that the inspection beam is directed to a first area which lies entirely within a second area previously flooded with charged particles of the flood beam.
According to one aspect of the present invention, an apparatus for simultaneously flooding a sample (e.g., a semiconductor wafer) to control charge and inspecting the sample is provided. The apparatus includes a charged particle beam generator arranged to generate a charged particle inspection beam substantially towards a first portion of the sample and a flood gun for generating a second beam towards a second portion of the sample. The second beam is generated substantially simultaneously with the inspection beam. The apparatus further includes a detector arranged to detect charged particles originating from the sample portion. In a further implementation, the apparatus further includes an image generator for generating an image of the first portion of the sample from the detected particles. In one embodiment, the sample is a semiconductor wafer. In another aspect of the present invention, a wehnelt electrode is attached to the flood gun to further control charge. The wehnelt electrode is provided with an independent voltage control for fine tuning the charge control process.
In an alternative embodiment, the invention pertains to a method of inspecting a sample. A first area of a sample is flooded with a flood beam to control charge on a surface of the sample. A second area of the sample is inspected with an inspection beam. The second area comprises at least a portion of the first area flooded by the flood beam. The inspection beam moves in tandem with the flood beam.
The present invention has several associated advantages. For example, since dissipation of charge on the sample is performed in-situ and simultaneously with the inspection, throughput for the entire process is significantly increased, as compared with a process that removes the sample from inspection to flood the sample. These and other features and advantages of the present invention will be presented in more detail in the following specification of the invention and the accompanying figures which illustrate by way of example the principles of the invention.